M1CROWAVE REFLECTOMETER

Related Applications

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 62/734,051, filed September 20, 2018, which is incorporated by reference.

Background

[0002] Non-destructive testing for cracks in metals and metallic objects is known. One technique for non-destructive testing is microwave reflectometry. A microwave reflectometer typically comprises a signal source generating a microwave frequency signal, an antenna, and a receiver. In such devices, a microwave signal is transmitted at an acute angle to a sample under test via the antenna. Microwaves incident on unflawed surfaces reflect array from the antenna, and microwaves incident on a crack or other surface flaw back-reflect to the antenna. The back-reflected wave may then be detected by a receiver in the microwave reflectometer.

[0003] One potential application for non-destructive crack detection is in a mill that produces hot-rolled cast steel strip. The size and angular orientation of cracks may provide valuable inpu ts to control the casting rolls of a casting mill. For example, transverse cracking (cracking direction is perpendicular to the rolling direction) may be due to an excess amount of pool turbulence, whereas longitudinal cracking (cracking direction is parallel to the rolling direction) may have different potential causes, including uneven shell solidification ln each case, determining size and orientation of cracking may provide inputs to real-time process controls to improve the quality of the strip steel product.

[0004] While microwave reflectometers have been proposed for detecting cracks in metal objects and surfaces, a disadvantage of known microwave reflectometry techniques is that it is difficult or impossible to detect crack length and angular orientation with a sufficient degree of precision and speed to identify potential causes of the cracking or to be useful as a control input for metal manufacturing operations. Another disadvantage is that known microwave reflectometers are not suited to a cast steel strip manufacturing environment due to very high temperatures involved in strip casting. What is needed is a device for non-destructively detecting the size and angular orientation of cracks in steel (or other metal surfaces) with a degree of precision sufficiently high to provide real-time manufacturing feedback control in a metal manufacturing environment.

Summary

[0005] A method for detecting defects on a surface of a metal product is provided. The method comprises directing at least one incident electromagnetic wave at a

measurement surface of the metal product, with at least one incident electromagnetic wave having a plurality of electric field vector directions and being within a spectrum spanning microwaves and millimeter waves, measuring electromagnetic waves reflected from the measurement surface on a first receive path to obtain a first electromagnetic wave complex value, measuring electromagnetic waves reflected from the measurement surface on a second receive path to obtain a second electromagnetic wave complex value, the first and second receive paths having different polarizations relative to each other, performing the steps of directing and measuring at a plurality of locations forming a two-dimensional measurement pattern comprising a plurality of first and second electromagnetic wave complex values, creating a first synthetic aperture radar image of the two-dimensional measurement pattern from the first electromagnetic wave complex values, creating a second synthetic aperture radar image of the two-dimensional measurement pattern from the second electromagnetic wave complex values, identifying a location of a defect from magnitude information derived from at least one of the first and second synthetic aperture radar images, determining defect size from magnitude information derived from at least one of the first and second synthetic aperture radar images, and determining an orientation angle of the defect by determining a difference between phase information derived from the first synthetic aperture radar image and the phase information derived from the second synthetic aperture radar image. The metal product may be a thin steel strip formed using a twin roll caster. [0006] The steps of creating one or more synthetic aperture radar images improves the signal-to-noise ratio of the plurality of first and second electromagnetic wave complex values and increases a resolution of the images.

[0007] The difference between the first and second phase information may be determined by subtracting phase information derived from the first synthetic aperture radar image from the phase information derived from the second synthetic aperture radar image. The step of identifying a location of a defect may further comprise multiplying the magnitude information derived from the first synthetic aperture radar image with magnitude information derived from the second synthetic aperture radar image.

[0008] The steps of directing and measuring may be performed at each of the plurality of locations using one or more antennas. The steps of directing and measuring may be performed at the plurality of locations using a plurality of antennas arranged in an array of antennas. The reflected wave may be measured through the plurality of the antennas simultaneously. The array of antennas may be a two-dimensional array. The array may comprise a plurality of subarrays of antennas.

[0009] The array of antennas may be a one-dimensional array and the metal product may be moved perpendicularly to a lengthwise axis of the array of antennas to form the two-dimensional measurement pattern. The reflected wave may be measured through the plurality of the antennas simultaneously to scan across the width of the metal product as the metal product is moving in the electromagnetic view of the array of antennas.

[00010] The at least one incident electromagnetic wave may comprise a first linearly polarized wave and a second linearly polarized wave orthogonal to the first linearly polarized wave to provide the plurality of electric field vector directions. The at least one incident electromagnetic wave may be elliptically polarized to provide the plurality of electric field vector directions.

[00011] The at least one incident electromagnetic wave may be circularly polarized to provide the plurality of electric field vector directions. The at least one incident electro magnetic wave may also comprise a plurality of electromagnetic waves including a right- hand circularly polarized electromagnetic wave and a left-hand circularly polarized electromagnetic wave to provide the plurality of electric field vector directions. An electromagnetic wave reflected from the defect may be linearly polarized, and the first receive path makes a right hand circularly polarized measurement of the back-reflected wave and the second receive path makes a left hand circularly polarized measurement of the back-reflected wave.

[00012] The angle of incidence of the directing and measuring steps with respect to a surface normal of the metal product may be 10° or smaller. For example, the angle of incidence of the directing and measuring steps may be substantially equal to the surface normal of the metal product.

[00013] The at least one incident electromagnetic wave may have a frequency of up to 300 GHz. For example, the at least one incident electromagnetic wave may have a frequency in a range of 26 GHz to 40 GHz. The at least one incident electromagnetic wave may comprise a plurality of electromagnetic waves of different frequencies for example, in a frequency range of 26 GHz to 40 GHz.

[00014] The metal product may be moved past an array of antennas and the plurality of electromagnetic waves are generated to keep up with the metal product speed.

[00015] A SAR microwave reflectometer for detecting cracks in metal surfaces may comprise a microwave signal source, at least one antenna coupled to the signal source, the antenna being configured to radiate a plurality of electric field vector directions, a receiver coupled to at least first and second receive paths, the first and second receive paths having different polarizations, and a SAR processor coupled to the receiver. The SAR processor may be configured to transform measurements received on the at least first and second receive paths into at least first and second SAR images, identify a location of a defect from magnitude information derived from at least one of the first and second SAR images, determine a defect size from magnitude information derived from at least one of the first and second SAR images and determine an orientation angle of the defect by determining a difference between phase information derived from the first SAR image and the phase information derived from the SAR image. The difference between the first and second phase information is determined by subtracting phase information derived from the first SAR image from the phase information derived from the second SAR image.

[00016] The at least first and second receive paths may comprise a dual polarized antenna. The at least first and second receive paths may comprise an array of dual polarized antennas. The array of antennas may be a one-dimensional array. The antenna array may be dimensioned to measure a width of a surface of a metal product. The metal surface may comprise a metal strip having a first surface and a second surface, and the at least one antenna comprises at least one first dual polarized antenna directed at the first surface and at least one second dual polarized antenna directed at the second surface.

[00017] The antenna may comprise a heat-resistant antenna body having at least one dual polarized aperture configured to be directed at a surface exhibiting high temperatures, and an electromagnetically transparent material shielding at least one dual polarized aperture from heat radiated from the surface. The antenna may be a waveguide antenna, and the body may comprise stainless steel, metal-lined ceramic, or conductive ceramic.

The electromagnetically transparent material may comprise a dielectric material or a ceramic material.

[00018] In one example, the dual polarized antenna of the SAR microwave

reflectometer may be a waveguide antenna comprising an antenna body having a first, second and third waveguide ports, the first and second waveguide ports being rectangular and the third waveguide port being square, a septum between the first and second waveguide ports, combining the first and second rectangular ports into the third port and an electromagnetically transparent material shielding at least the third port from heat.

Brief Description of the Drawings

[00019] Figure 1 is a block diagram of a multiplexed antenna array according to one aspect of the present invention. [00020] Figure 2 is a block diagram of a SAR microwave reflectometer according to another aspect of the present invention.

[00021] Figure 3 is illustration of an antenna array portion of a SAR microwave reflectometer relative to a metal surface being measured according to another aspect of the present invention.

[00022] Figures 4a and 4b are illustrations of a dual polarized waveguide antenna, as one example, which may be used to implement the present invention.

[00023] Figure 5 is a flow chart for detecting a location, magnitude, and angular orientation of cracks according to another aspect of the present invention.

[00024] Figure 6 is a flow chart of a SAR omega-k algorithm according to another aspect of the present invention.

Detailed Description

[00025] A robust, non-destructive, Synthetic Aperture Radar (SAR) microwave reflectometer and method of processing reflected microwaves to achieve crack detection with precise angular orientation and size values is provided. A SAR microwave

reflectometer according to one example of the present invention comprises a microwave signal source, one or more antennas, a receiver, and signal processing components to convert reflected and received microwave signals into SAR images of a surface under test.

[00026] In SAR imaging, multiple microwave measurements are performed over a surface and then combined to form an image. Typically, the SAR antenna is moving across the surface as transmission and reception occur, and the combination of the received signals builds a synthetic aperture that is much longer than the physical antenna width.

The same principles apply when the antenna is stationary and a surface to be imaged is moving with respect to the antenna or antennas.

[00027] lmaging of a sample may be performed using a single antenna that is scanned over a two-dimensional grid over a sample, a one-dimensional array of antennas that are scanned linearly across the sample, or a two-dimensional array of antennas positioned over a stationary sample. A single antenna within the array can act as a transmitter only, a receiver only, or both. The number of transmitters and receivers in the array does not have to be equal. Surfaces that may be sampled include, but are not limited to, metal surfaces, such as cast strip metals, conductive ceramics (refractory), non-conductive dense ceramics, carbon fiber composites, rubber, and other materials with a high dielectric or impedance contrast to air.

[00028] Referring to Figure 1 an array of antennas 12 may be multiplexed into a microwave reflectometer 10 with a multiplexer 14, such as microwave switches, signal modulation, and the like. Multiplexing may be performed at microwave frequencies, intermediate frequency (IF) stage (after down mixing). An alternative to multiplexing is using one receiver (down-conversion and IF stage) per antenna.

[00029] Electromagnetic waves may be polarized. Polarization is defined as the path the tip of electric field vector traverses as a function of time at a fixed location. The most general case of polarization is elliptical. A circularly polarized wave is a specific case where the electric field vector has a constant magnitude and, at a given point in space, rotates about a circle as time progresses. A circularly (or elliptically) polarized wave can be in one of two possible states, right circular polarization in which the electric field vector rotates in a right-hand sense with respect to the direction of propagation, and left circular

polarization in which the vector rotates in a left-hand sense. A linearly polarized wave is another specific case where the electric field vector does not rotate. Linearly polarized waves may be vertical, horizontal, or angles in between with respect to a reference direction. A pair of linearly polarized waves that are orthogonal to each other may be referred to as cross polarized waves.

[00030] It has been found that to accurately determine the direction of cracks, two orthogonal polarizations are advantageous. Linear features (e.g., cracks) scatter

electromagnetic fields in a single linear polarization. To maximize the chance of detecting a crack, the polarization of the wave should be orthogonal to the lengthwise angular orientation of the crack. Because the angular orientation of cracks in cast steel strip may occur in transverse, longitudinal, and intermediate directions, the present invention may advantageously transmit one or more circularly polarized waves so that all directions are scanned for cracks. In one example, both left and right hand circularly polarized waves are transmitted during scanning. This maximizes the likelihood for detecting a crack.

[00031] Referring to Figure 2, a more detailed block diagram of one example of a microwave reflectometer 110 is provided. The present invention is not limited to this example. The signal source may comprise a frequency synthesizer 114, a first power amplifier 116, a directional coupler 118, and a second power amplifier 120. In another example, an isolator takes the place of the second power amplifier 120. The signal source may generate one or more microwave signals in a frequency range of up to 300 GHz. The frequency synthesizer may receive a reference input of, for example, 100 MHz and output frequencies in the range of 20 -40 GHz. In one example, a plurality of microwave signals are generated in a frequency range of 26 GHz to 40 GHz. The directional coupler 118 applies a portion of the amplified signal to the second power amplifier 120, and a portion of the amplified signal as a phase reference to one or more down mixers on the receiver path.

The second power amplifier 120 or isolator prevents signals reflected from the surface under test from reflecting back into the signal source. Optionally, a frequency doubler 122 may be included after the amplifier and before the directional coupler, and the frequency synthesizer may provide an output in the range of 10-20 GHz.

[00032] A heterodyne receiver is one example of a receiver that may be used in a microwave reflectometer. A heterodyne receiver may comprise a down conversion stage 132 (using mixers 132a) followed by an intermediate frequency (IF) stage 134. The IF stage 134 contains signal conditioning (e.g., filters). A digital signal processing (DSP) 136 unit which provides vector measurement. Analog circuits may be used instead of a DSP unit.

[00033] In the example of Figures 2 and 3, four dual-polarized antennas 142 are arranged in a linear array 140. The receive path includes eight down mixers 132a. Each of the down mixers 132a may have a radio frequency (RF) input coupled to one polarization of one of the antennas 142, a local oscillator (LO) input coupled to a synthesizer 144, and an intermediate frequency (IF) output coupled to an IF receiver. The synthesizer 144 of the receiver should be synchronized to the synthesizer 114 for the signal source. In one example, both synthesizer 114 and synthesizer 144 are coupled to a common frequency reference 148. In another example, one of the ports of one of the antennas in the array is coupled to the second power amplifier / isolator of the signal source, and that port is used as a transmit port, and an output of the directional coupler is provided to the RF input of one of the eight down mixers as a phase reference.

[00034] The reflected electromagnetic wave information may be represented as complex values in the frequency domain. The received signals may also comprise a measured in-phase (I) component and a measured quadrature (Q) component.

[00035] The example of the microwave reflectometer 110 as illustrated in Figure 2 has one transmit antenna and multiple receive antennas. In some embodiments, the microwave reflectometer employs multiple transmitters and receivers, for example, an equal number of transmitters and receivers. This allows for better coverage of the strip independent of the distance between the array and the strip. In order to operate multiple transmitters simultaneously without interference, modulation is required

[00036] Various methods for modulating the transmitter signals may be

implemented. In one example, a frequency diversity scheme similar to Frequency Division Multiple Access (FDMA) is employed. In this example, for each of transmit antennas TXi through TX _n , each transmitted signal is at a unique IF frequency. On the receive path, in one example, the calculated combinations are limited to the signals by the two nearest neighbors at each side. For example, a receive path including receive antenna RXk would process frequencies transmitted by transmit antennas TXk-2 through TXk ₊₂ . The number of frequencies processed may be varied to included more or less combinations depending on the antenna beamwidth and distance of the antenna array to the cast strip. Bandpass filters corresponding to the transmitted frequencies may be used to reject out of band noise. In one example, operations are performed synchronously for all channels every 5 ps for each frequency point. [00037] In another example, code modulation based on Walsh codes is employed. A single IF frequency is used for all channels. The IF frequency (e.g., 40 MHz) should be lower than f _s /2, where f _s is the Analog to Digital Converter (ADC) sampling rate. Each transmitter signal is modulated by a one Walsh code (Wk). A family of codes are generated for all transmitters in one unit (operated by one digital system). Code length is at least N+2, where N is the number of channels (transmitters). Once again, on the receive path, the calculated combinations are limited to the received signal by two nearest neighbors at each side. The number of codes processed may be changed to included more or fewer combinations depending on the antenna beamwidth and distance of antenna to strip.

[00038] In one example, the SAR microwave reflectometer 110 is adapted to detect cracks in cast steel strip 150 produced by a twin roller casting process as the steel is being produced. Referring to Figure 3, a one-dimensional array of antennas is oriented with its lengthwise axis perpendicular to the direction of travel of the cast strip 150. The antenna array 140 may be spaced, for example, at about 100 mm from the cast strip. The antenna array 140 may be dimensioned to be able to image the entire width of the cast strip as it travels past the antenna array 140. One antenna array 140 may be located above the cast strip 150 and another antenna array 140 may be located below the cast strip 150 so that both top and bottom surfaces of the cast strip 150 may be imaged. In one example four antennas 142 are arranged in a linear array. Additional examples have different numbers of antennas and may comprise subarrays of antennas.

[00039] Each antenna 142 may comprise a dual-polarized antenna. Referring to Figures 4a and 4b, in one example, the dual polarized antenna is a three-port antenna configured to transmit and receive left-hand circular and right-hand circular polarizations. Figure 4a has a top portion removed so that the interior waveguides are visible. For example, a waveguide antenna may have an antenna body having a first waveguide port 162, second waveguide port 164 and third waveguide port 166. The first and second waveguide ports 162, 164 may be rectangular and coupled to the signal source and/or receiver. The third waveguide port 166 may be square and be directed at the sample under test. The third port may be a transmitting/receiving port that is directed at a sample under test. A stepped septum 168 may be provided between the first and second waveguide ports, combining the first and second rectangular ports into the third port to provide left- hand and right-hand circular polarization.

[00040] ln one example, the waveguide antenna 142 may be used to image cast steel strip 150 shortly after casting and before cooling. The surfaces of such materials radiate considerable heat. The waveguide antenna 142 may be adapted for use in such a high heat environment by using heat resistant materials and including an electromagnetically transparent material shielding at least the third port from heat 166. The

electromagnetically transparent material may comprise a dielectric material filling at least the third port. The electromagnetically transparent material may also comprise a ceramic material that covers an opening of at least the third port. The antenna body may be formed from stainless steel, conductive ceramic, or ceramic material with the waveguide ports being metal lined.

[00041] A method 170 of performing measurements in illustrated in Figure 5. In step 172, at least one incident electromagnetic wave is directed at the surface under test. The angle of incidence of the electromagnetic wave with respect to a surface normal of the surface under test may be 10° or smaller. In one example, the angle of incidence maybe equal to the surface normal. In another example, the angle of incidence may be selected to avoid receiving a specular reflection of the incident wave, and may be 10° to 45° or larger with respect to the surface normal. Where the angular orientations of the cracks are unknown, the incident electromagnetic wave should have more than one electric field vector direction. For example, the incident electromagnetic wave may comprise two linearly polarized electromagnetic waves, with the two waves having different polarization to each other. For example, the different polarizations may be, but are not required to be, orthogonal to each other. In another example, the incident electromagnetic wave may be elliptically polarized to provide the plurality of electric field vector directions to allow for detecting cracks at any angle. In another example, the incident electromagnetic wave may be a left or right circularly polarized wave to provide the plurality of electric field vector directions. In another example, the incident electromagnetic wave may comprise a right- hand circularly polarized electromagnetic wave and a left-hand circularly polarized electromagnetic wave to provide the plurality of electric field vector directions. Right hand circularly polarized and left hand circularly polarized electromagnetic waves are another example of orthogonal polarization.

[00042] Electromagnetic waves reflected from the sample under test may include scattered signals from a surface defect, such as a crack. An electromagnetic wave reflected from a linear defect such as a crack may be linearly polarized in a direction perpendicular to the length of the crack. The reflected waves are received by the receiver and are measured in step 174. The receive paths for the waves include at least first and second receive paths, with the first and second receive paths having different polarizations and, preferably, orthogonal polarizations. In one example, the first receive path makes a right hand circularly polarized measurement of the reflected wave and the second receive path makes a left hand circularly polarized measurement of the reflected wave. Orthogonal linear polarized receive paths may also be used. The measured reflected waves are represented as a vector having a magnitude and phase or a complex value having real and imaginary parts.

[00043] Measurements may be made periodically or continuously as the strip steel passes by the one-dimensional array, providing a two-dimensional set of measurements. This two-dimensional set of measurements may be processed into SAR images, as described in more detail below.

[00044] The angle of incidence of the incident electromagnetic wave and receive path for the reflected electromagnetic wave with respect to a surface normal of the surface under test may be 10° or lower. At least one incident electromagnetic wave may have a frequency of up to 300 GHz. For example, the incident electromagnetic wave may have a frequency in a range of 26 GHz to 40 GHz. The incident electromagnetic wave may also comprise a plurality of electromagnetic waves of different frequencies, which may be in the range of 26 GHz to 40 GHz. [00045] It has been found that transforming the measured electromagnetic waves to synthetic aperture radar (SAR) images improves the signal-to-noise ratio of the plurality of the electromagnetic wave complex values, and improves the resolution of the surface measurements. Accordingly, the received signals are provided to a processor for SAR processing. The SAR processor transforms the measured received signals from the frequency domain to the spatial domain. The output of the SAR processor comprises complex values with x, y coordinates corresponding to a location on the measured surface, and a z coordinate corresponding to a height of the surface.

[00046] Referring to figure 6, in one example, the SAR algorithm that may be used in the present invention is an Omega-k algorithm. Application of the Omega-k algorithm to SAR image processing is well known to persons of skill in the art and will not be explained in detail. Received data is input to the spectral estimator block, which may comprise a two- dimensional Fast Fourier Transform. A function may be applied to connect from 2D frequency domain to a 3D domain. As illustrated in Figure 6, a non-uniform FFT (NUFFT) may be used in place of Stolt interpolation. Range refers to the direction normal to the surface of the cast strip. Cross range refers to the length and the width of the cast strip.

[00047] Continuing with the example for use in a cast steel strip environment, SAR image data is generated for each of the right-hand and left-hand circularly polarized measurements in step 176. The SAR image data comprises complex values (real plus imaginary values) representing the electromagnetic (microwave) reflectivity of the surface of the metal. The complex values of the SAR image data may be further transformed into phase information and magnitude information.

[00048] A surface defect may be detected and its location identified from magnitude information derived from the complex values of at least one of the SAR images in step 178. The magnitude information represents the intensity of the reflected signal from the sample. In the absence of a crack, the magnitude will be uniform. The presence of a crack will cause a localized change in measured intensity, and is detectable in the magnitude information. The larger the crack, typically the higher the change in magnitude at the locations corresponding to the crack. Accordingly, the size of a crack or other surface defect may be determined from magnitude information. In one advantageous application of the present invention, determining a size and location of a surface defect may be improved by multiplying the magnitude information derived from the complex values of the SAR images from the two different receive polarizations.

[00049] When receiving two different polarizations, the SAR images from the two different receive polarizations may also be used to determine an angular orientation of the defect. The angular orientation of the defect may be determined using a difference between phase information derived from one SAR image relative to the other SAR image at the location of the detected crack. The difference between the phase information of the two images may be determined by subtracting phase information derived from the complex values of one SAR image from the phase information derived from the complex values of the other SAR image in step 180.

[00050] The magnitude and phase information derived from the SAR images may also be transformed into visual images to provide a visual representation of the measured data. For example, magnitude and phase information may be mapped to a Jet color map, a greyscale color map, or other color map. In the example of a Jet colormap, the minimum value may be mapped to blue and the maximum value may be mapped to red.

[00051] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. For example, the methods described herein may be performed with less or more steps/acts or the steps/acts may be performed in differing orders. Additionally, the steps/acts described herein may be repeated or performed in parallel with one another or in parallel with different instances of the same or similar steps/acts. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.